The present invention relates to an ultra dense composite strip for flexible pipe armour, of the type comprising longitudinal rovings having high mechanical properties, such as carbon fibres, placed within a matrix of thermoplastic or thermosetting material, in the form of a flat tape of approximately rectangular cross section.
Documents FR 2,739,673 A and FR 2,739,674 A, the teaching of which is incorporated here for reference, disclose armour based on a woven strip, for flexible pipe, especially pipe used in deep sea applications in the oil industry. This strip is preferably based on aramid resin (xe2x80x9cKevlarxe2x80x9d) fibres, although other fibre materials are not excluded. Moreover, strips are also known which are made of a fibre-reinforced composite plastic (fibre-reinforced plastic or FRP strips) consisting especially of rovings of juxtaposed parallel fibres contained in a matrix of thermoplastic material (such as a polyamide) or thermosetting material (such as an epoxy resin), the said matrix being continuous longitudinally and transversely to the axis of the rovings, the matrix embedding each filament in a continuous manner so as to hold the filaments together. The fibres used are generally glass or aramid fibres. However, owing to the drawbacks encountered, for example, with glass fibres under certain extreme operating conditions (the relatively large thickness of the tapes obtained possibly resulting in fibre fractures, causing partial or complete failure of the armour under certain bending and/or torsional stresses), the idea of using reinforcing fibres having high mechanical properties, such as ceramic or carbon fibres, was born, these fibres, and especially carbon fibres, being renowned for their high tenacity and their great chemical inertness, carbon fibre being moreover the most accessible of the fibres having high mechanical properties.
However, the use of fibres such as fibres having high mechanical properties, such as carbon fibres, for flexible pipe armour gives rise to difficulties which it is the objective of the present invention to solve.
This objective is achieved within the context of the invention by means of an ultra dense composite strip for flexible pipe armour, of the type comprising longitudinal rovings of fibres having high mechanical properties, such as carbon fibres, placed within a matrix of thermoplastic or thermosetting material, in the form of a flat tape of approximately rectangular cross section, at least one of the facesxe2x80x94the upper or lower facexe2x80x94of which is reinforced by a film fastened to the said face over approximately its entire width and over approximately its entire length.
This is because it was noticed that, by this apparently simple means, it is possible to remedy the lack of transverse shear strength (tear strength) characteristic of an ultra dense flat tape of the carbon-fibre-based type in question, having a small thickness (typically less than 1 mm) so as to be able to be wound around a cylindrical core, the diameter of which may be only of the order of 100 mm. There is a risk that the torsional and bending stresses imposed on such a thin flat strip when winding it to a small diameter, or when the flexible pipe undergoes axial compression, initiate longitudinal tears within the strip, tears which easily propagate throughout the strip. The film of the invention prevents the tearing from being initiated, or at the very least from propagating. It therefore allows the strip to be wound to smaller diameters and improves the buckling resistance of the armour (during the phenomenon commonly called the inverse or compressive end cap effect due to the effect of the external pressure exerted on the ends of the flexible pipe). The thickness of the film according to the invention is of the order of one tenth of that of the strip.
The film covers the said face continuously or even discontinuously over its length, and continuously over its width.
The film may be a plastic film, either a homogeneous film or one containing fibres which may be short and form a discontinuous reinforcement (a mat) or which may be long and form a continuous reinforcement (an impregnated woven or an impregnated nonwoven). The fastening of the film is obtained by sheathing completely around the strip (the sleeve being adherent or non-adherent) or, when the fastening involves only certain faces of the strip, it takes place by adhesive bonding, or by simultaneous lamination with a thermoplastic or thermosetting material during the process of manufacturing the strip.
The tape may be covered with several thicknesses of films, the films being identical or different.
When the film is or contains a woven, the two main directions of the woven are advantageously placed obliquely with respect to the longitudinal direction of the tape, preferably making an angle of between 30xc2x0 and 65xc2x0 with this direction.
The fibres incorporated into the film and/or forming the woven are fibres chosen for their specific properties and especially their chemical resistance, most generally from the following materials: carbon, glass, aramid resin.
The invention will be more clearly understood by means of the following description, with reference to the appended drawings in which: